One of the most costly aspects of IC technology is bonding the IC chip to the rest of the circuit package. A proposed solution to this problem comprises automated bonding of metal beam leads to the IC chip, thus avoiding manual wire bonding. Such a technique uses a roll of a laminate that carries to a bonding station a plurality of frames each of which has many generally planar microscopic, spider-shaped metal fingers known as beam leads. At the bonding station the inner portions of the beam leads are aligned and then simultaneously connected to the bonding sites on the IC chip. The outer portion of the beam leads is then available for bonding to remaining portions of the circuit package.
Such beam leads can be manufactured by a photoresist process. U.S. Pat. No. 4,247,623 discloses such a process and a blank or laminate for use therein having a structure comprising a flexible strip of electrically conductive metal, a layer of positive-working resist adhered to one surface of the metal strip, and a layer of negative-working resist adhered to the opposite surface of the metal strip. The process includes the steps of imagewise exposing and developing the positive-working resist and the negative-working resist. The developed positive resist forms a protective coating on the beam lead portions of the metallic layer. The exposed metallic layer is subsequently etched to form the beam leads. The developed negative resist forms a window-bearing support spacer holding the beam leads in the desired orientation. The beam leads are maintained in their proper orientation prior to and during bonding by the spacer formed from the negative-working resist.
It is desirable from the standpoint of electronic reliability that the negative-working resist and support spacer formed therefrom exhibit good dimensional integrity and hardness at temperatures of 200.degree. C. and higher. The reason for this is that the function of the negative-working resist of supporting the leads prior to, during, and after bonding of the IC chip can be negated by poor dimensional integrity and/or softness. Processing at temperatures of at least 200.degree. C. frequently takes place, for example, during the steps of curing the second resist to effect complete polymerization thereof and bonding the inner and outer lead portions.
While the laminate described in U.S. Pat. No. 4,247,623 provides, under most conditions, superior beam leads, its performance at processing temperatures of 200.degree. C. and higher has not been entirely satisfactory. Thus, what has been needed is a laminate, having the advantages, i.e. good adhesion and flexibility, of the laminate described in U.S. Pat. No. 4,247,623, and which exhibits improved dimensional integrity and hardness at processing temperatures of 200.degree. C. and higher.